At 11:20 p.m., a hydrogen skid was placed into service in a nuclear power plant, outside the controlled area. This skid, like seven others, was located in a gas cylinder storage facility. It was supplied by several hydrogen cylinders at a pressure of 200 bar. As its pressure was rising, a trap on the skid was blown off, causing the hydrogen to leak and ignite. The flaming jet measured 40 cm and continued as the system was open. Surprised by the incident, an employee fell and slightly injured his arm. The site’s internal emergency plan was initiated.

The storage facility was located just 20 m from a 400 kV high-voltage power line. The use of hydraulic means was therefore impossible. Firefighters were able to close the hydrogen supply valve, which stopped the leak. The alert was called off at 2 a.m. The two skids impacted by the fire were removed from service.

The analysis of the accident showed that the trap was loose and that its threads were worn. Mechanical rubbing during the expulsion had released enough energy to ignite the hydrogen. The operator conducted electrical and mechanical inspections of all the site’s gas storage facilities. It was also noted that 60% of the traps had tightening defects.

The operator took the following corrective actions:

• periodic inspection of the tightening of the traps and their threads;
• review of the gas storage facility’s access conditions;
• study of modifications to the systems for purging flammable gas skids and protection against self-ignition;
• thoughtful analysis of the leak search procedure: study to replace the hydrogen detection system via an explosimeter with compressed air and a microbubble search.

The operator issued rapid feedback on the accident to other nuclear power plants. In this document, it described the accident and the safety measures to be taken to prevent it.